The present invention relates, in particular, to a display device of active matrix type having a charge pump booster circuit formed on a substrate surface of a display panel.
Portable devices such as portable telephones and digital still cameras are driven by batteries. These portable devices include devices that need a voltage higher than the battery voltage. Therefore, a high voltage is generated by a booster circuit in the devices.
In general, a charge pump booster circuit is used when a consumed current of a device requiring a high voltage is small.
A small-sized liquid crystal display device included in a potable device typically needs a voltage higher than the battery voltage or a voltage of negative polarity. For a voltage corresponding to a small current consumption, the above-described charge pump booster circuit is used.
For obtaining a high voltage, a charge pump voltage doubling booster circuit is used. For obtaining a potential of negative polarity, a charge pump inversion booster circuit is used.
In general, the charge pump booster circuit includes stabilizing capacitance for stabilizing output potential, pumping capacitance for storing charge on the stabilizing capacitance (pulling charge out from the stabilizing capacitance), and a plurality of switching elements for controlling the stabilizing capacitance and the pumping capacitance.
The charge pump booster circuit conducts driving by repeating two time periods (for example, A and B). In the case of the voltage doubling booster circuit, a first terminal of the pumping capacitance is connected to an input voltage VCC and a second terminal of the pumping capacitance is connected to GND for the time period A. Subsequently, for the time period B, the first terminal of the pumping capacitance is electrically disconnected from VCC, and then the second terminal of the pumping capacitance is connected to VCC. As a result, a potential at the first terminal of the pumping capacitance becomes twice as high as VCC. In this state, the first terminal of the pumping capacitance is connected to the stabilizing capacitance to store charge on the stabilizing capacitance. Thereafter, the first terminal of the pumping capacitance is electrically disconnected from the stabilizing capacitance, and then the time period A is repeated.
By thus repeating the time periods A and B, charge is stored on the stabilizing capacitance and ideally it is possible to obtain an output voltage twice as high as VCC.
In the case of the inversion booster circuit, a first terminal of pumping capacitance is connected to GND and a second terminal of the pumping capacitance is connected to VCC in a time period A. Subsequently, in a time period B, the first terminal of the pumping capacitance is electrically disconnected from GND, and then the second terminal of the pumping capacitance is connected to GND. As a result, a potential at the first terminal of the pumping capacitance becomes −1 time as high as VCC. In this state, the first terminal of the pumping capacitance is connected to the stabilizing capacitance to pull out charge from the stabilizing capacitance. Thereafter, the first terminal of the pumping capacitance is electrically disconnected from the stabilizing capacitance, and then the time period A is repeated. By thus repeating the time periods A and B, charge is pulled out from the stabilizing capacitance and ideally it is possible to obtain an output voltage that is −1 time (inverted) as high as VCC.
For increasing an output current of such a charge pump booster circuit, it can be coped with by raising the repetition frequency of the time period A and the time period B and using large pumping capacitance.
In JP-A-2002-291231, a circuit configuration in the case where the charge pump booster circuit is used in a liquid crystal display device is disclosed. In general, the current consumption changes largely according to the display state in liquid crystal display devices. Therefore, an application example of the charge pump booster circuit described in JP-A-2002-291231 has a feature that it estimates a current consumption in the liquid crystal display device and optimally adjust an operation frequency (the number of times of repetition of the time period A and the time period B) of the charge pump booster circuit by monitoring an output voltage of the charge pump booster circuit. As a result, a power supply circuit that can reduce the power consumption loss at ordinary times when the current consumption is low while coping with a maximum current consumption in a specific display pattern is implemented.
A switched capacitor stabilized power supply apparatus described in JP-A-2003-23770 includes a booster circuit including pumping capacitance C1 and switching elements SW1 to SW4. Charging and discharging the pumping capacitance C1 is changed over by switching operation of switching elements SW1 to SW4. At the time of discharging the pumping capacitance C1, a DC voltage Vin applied to an input terminal IN is boosted and output. In this switched capacitor stabilized power supply apparatus, the DC voltage Vin is divided by resistors R1 and R2 to monitor the output voltage Vin.